[Pw_forum] Fw : ev.x procedures

[Sandro Scandolo]

Dear Bertrand,

pressure is not an input of your pw calculation. It is an output of the 
ev.x calculation.

What ev.x does is to fit the results of your E(V) pw calculations to a 
chosen EOS functional form, with a number of parameters. Most functional 
forms (e.g. Birch-Murnaghan) include the parameters K and K' (see 
http://en.wikipedia.org/wiki/Birch-Murnaghan_equation_of_state ). P(V) 
can then be calculated from the chosen E(V) (and the fitted parameters) 
as P(V)=-dE(V)/dV .

Notice that the P(V) calculated form the EOS with the fitted parameters 
may be slightly different with respect to the one calculated by pw.x at 
the volumes where you did your pw.x calculations. But this is another 
story (related to 
http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress 
).

So, coming to your question, K' is not obtained by finite differences, 
but rather directly as a result of the E(V) fit.

Sandro


Bertrand SITAMTZE wrote:
>
>
> --- En date de : *Mar 2.2.10, Bertrand SITAMTZE /<siyouber at yahoo.fr>/* 
> a écrit :
>
>
>     De: Bertrand SITAMTZE <siyouber at yahoo.fr>
>     Objet: ev.x procedures
>     À: "pw code" <pw_forum at pwscf.org>
>     Date: Mardi 2 Février 2010, 11h04
>
>     (SORRY, THERE WAS A SMALL MISTAKE IN MY PREVIOUS MESSAGE)
>
>
>     Dear all,
>
>     As I know, scf and nscf in pw.x are with zero pressure (please
>     correct me if I am wrong). Thus, when evaluating the EOS
>     parameters with ev.x, the code uses the E vs VOLUME data to
>     evaluate the bulk modulus throught the second derivative. But, how
>     does the code proceed in order to evaluate the pressure derivative
>     of the bulk modulus K', without varing the pressure? If otherwise,
>     can somebody tell me the range of pressure used to evaluate K'?
>
>     Please, note that K' is defined as K'=(dK/dP) at P=0.
>
>
>     Thanks very much for your answers.
>
>
>
>     ***************************************
>     Bertrand SITAMTZE YOUMBI
>     Laboratory of Material Sciences
>     University of Yaounde I-Cameroon
>     *************************************
>
>
>
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-- 
Sandro Scandolo, ICTP | http://www.ictp.it/~scandolo